Recent concern over the state of the environment, the dangers of high levels of greenhouse gas emissions and global warming has sparked a recognition of the need to develop clean renewable energy sources. Biomass gasification of abundant organic materials such as wood chips, forest residue and farm waste is a leading source of clean renewable energy that, in addition to other constituents, can result in a synthesis gas that can contain up to 19% hydrogen (H2) and 20% carbon monoxide (CO).
By chemically processing such constituent materials, useful gases may be recovered and used to produce various forms of energy, including electrical and mechanical energy. This electrical energy may be used to power homes, industrial buildings, and farm and industrial machinery located far away from a power grid.
Hydrogen fuel cells are a promising technology for use as electrical power sources. With only water as by-product and no greenhouse gases emissions, hydrogen fuel cells provide considerable environmental benefits. Fuel cells receive hydrogen as an input and return electrical energy that may be used in numerous applications. Hydrogen for fuel cells can be produced in several ways, such as through fossil fuel reformation, the steam-iron process, thermochemical water splitting, and water electrolysis.
Fossil fuel reforming accounts for approximately 95% today's hydrogen production, and is a chemical process in which steam reacts at a high temperature with a fossil fuel inside of a “reformer” to produce hydrogen and carbon monoxide. Reforming typically occurs in the presence of a metal-based catalyst and at high temperatures of 700 to 1100° C. This process can be applied to a large range of hydrocarbon feedstocks, including propane, gasoline, autogas, methanol, diesel fuel, and ethanol. However, this process is limited to the availability of fossil fuels, which will increase in cost and decrease in availability in the future.
The steam-iron process is one of the oldest known methods for producing hydrogen in which coal is gasified to a lean reducing gas made up of hydrogen and carbon monoxide. This gas then reacts with iron oxide, typically such as magnetite (Fe3O4), to produce wustite (FeO) and/or iron metal (Fe). Then, the wũstite and/or iron metal is re-oxidized with steam to form magnetite and H2 gas.
The steam-iron process takes place at temperatures ranging from 600 to 900° C., but may occur at lower temperatures when the reaction takes place in the presence of catalysts such as transition metal or potassium hydroxide. Further, the steam-iron process may also occur at lower temperatures when the reactive surface area of the iron-bearing water-reducing material is increased through processes such as grinding. However, the availability and price of coal, as well as use of a fossil fuel, remain a large drawback.
In thermochemical water splitting, the intense heat required to split water into hydrogen and oxygen is typically derived from concentrated sunlight or recycled waste heat from a nuclear reactor. Consequently, this process involves near-zero greenhouse gas emissions, but remains under development to identify reactor designs, systems, and materials that will be cost-efficient and durable. Therefore, a commercially viable thermochemical reactor is as of yet unavailable.
Similarly, conventional water electrolysis involves the splitting of water into hydrogen and oxygen via an electric current, but is very expensive and consumes high amounts of energy in comparison to fossil fuel-based processes. As an improvement to conventional water electrolysis, to reduce the amount of electrical energy required to facilitate water splitting, a method of supplying natural gas to the electrolyzer has been proposed, as reflected in U.S. Pat. No. 6,051,125.
However, this method requires fossil fuel consumption, as well as monitoring of electrodes that may become contaminated with carbon deposited by reaction of natural gas with oxygen. An alternative method of electrolyzing high-temperature steam at a high-temperature of 800° C. or higher. In this method, high levels of thermal energy are substituted for the high levels of electrical energy typically required to electrolyze water, thereby lowering reducing the electrical power required for water electrolysis. But, the required thermal energy is often derived from fossil fuels.
For hydrogen to be used in hydrogen fuel cells, a high degree of purity is critical as even trace impurities present in the H2 can poison the anode, membrane, and cathode of the fuel cell, resulting in reduced and inefficient performance. In order to efficiently produce ultraclean hydrogen that may be effectively used to generate electricity, carbon monoxide levels must be kept to an absolute minimum, particularly at less than 10 parts per million, to ensure that a hydrogen fuel cell remains efficient and functional.
Therefore, as hydrogen produced by the available known methods typically either includes unacceptable levels of impurities or, to produce H2 of sufficient purity or requires excessively high levels of energy input, there remains a need for a method of producing high purity hydrogen at low cost without greenhouse gases emissions.